#hyphalosaurus

Fish of the yixian formation

Polycephaly, the condition of having more than one head, is seen occasionally in modern animals. But due to its overall rarity, and the low chance of any given individual being fossilized, the odds of finding any ancient examples are incredibly low.

Crocodilians have long been the dominant aquatic reptiles on Earth, but they are by no means unique; they’ve been sharing their environments with other reptiles for hundreds of millions of years.  Some of these other aquatic reptiles lived more recently than you might think...

Champsosaurus, the animal pictured above, might look like a crocodilian, but it actually belongs to an entirely different order of reptiles - the order Choristodera.  Paleontologists are uncertain how to classify these animals, but the current leading theory is that the choristoderes are archosauromorphs - not true archosaurs, but more closely related to them than to other types of reptiles.  (Image by Nobu Tamura.)

The earliest known choristoderes lived during the Early Jurassic (although fragmentary remains indicate that they might have lived in the Triassic as well).  These early forms were small and lizard-like, and already well-adapted to life in the water.  One such form, the Late Cretaceous Hyphalosaurus of China (pictured above), is one of the world’s most completely known fossil animals; thousands of specimens have been found, representing all growth stages from egg to adult.  (Image by Matt Martyniuk.)

One Hyphalosaurus specimen, in a one-in-a-million stroke of paleontological luck, was preserved with two heads, making it the oldest known case of polycephaly.

In addition, Hyphalosaurus and its close relatives gave birth to live young.  In that respect, they were unique among freshwater aquatic reptiles of the Mesozoic.

(Above: Champsosaurus, by R.J. Palmer.)

Later choristoderans became much larger - between five and ten feet in length - as well as more crocodilian in appearance.  Their long snouts and strong jaws were adaptations for catching small, fast-moving prey underwater.  These animals, called champsosaurs, were so specialized for aquatic existences that they were all but incapable of going on land.  Only females could climb ashore to give birth; males could not leave the water.

The choristoderans survived the Cretaceous extinction and became widespread in North America, Europe, and Asia.  Seven valid species of Champsosaurus are known from the United States, Canada, Belgium, and France.

GUYS LOOK IT'S ONE OF THE EARLIEST EXAMPLES OF DICEPHALUS 

THIS IS A HYPHALOSAURUS, A DIAPSID REPTILE FROM THE EARLY CRETACEOUS

The submerged lizard, Hyphalosaurus (1999)

Phylum : Chordata Class : Reptilia Order : Choristodera Family : Hyphalosauridae Genus : Hyphalosaurus Species : H. lingyuanensis, H. baitaigouensis

Early Cretaceous (123 - 122 Ma)

80 cm long (size)

Liaoning province, China (map)

Two specimens of Hyphalosaurus have been described with clear impressions of the skin. One specimen represents H. lingyuanensis, and the other (with clearer impressions) cannot be assigned to a species because part of the neck (the length of which is a key indicator of species) was destroyed when fossil dealers grafted a skull from a different specimen onto the slab. However, both specimens show nearly identical patterns of scales.

Hyphalosaurus was covered mostly in small, irregularly patterned polygonal scales, though these varied across the body. The scales of the hind legs were smaller, finer and more irregular than those of the torso, while the scales of the tail were nearly square and arranged in more regular rows. In addition to the small scales, two rows of large, round scutes with shallow keels ran along the animals sides. One row ran directly along the flank, with the other either slightly higher or lower and composed of scutes only 1/4 the size of the flank scutes. The flank row of larger scutes extended all the way to the base of the tail, and remained uniform in size across the entire row.

The tail itself has preserved soft tissue extending well beyond the margins of the skeleton. This, combined with the already flattened appearance of the tail vertebrae, suggests that a ridge of skin may have extended from the top and bottom of the tail creating a small fin. The feet and hands also appear to have been webbed.

Both Hyphalosaurus species were aquatic, a lifestyle reflected by their long necks and tails and relatively small limbs. Superficially, they resembled miniature plesiosaurs, though this resemblance arose convergently and does not reflect a close relationship. Hyphalosaurus was among the most aquatically adapted choristoderans, with smoother, flatter scales than its relatives, a tall and flattened tail for swimming, a long neck and webbed feet. Because the torso was fairly inflexible and the limbs were not particularly adapted for aquatic life, Hyphalosaurus probably swam using mainly its tall, flattened tail. The chest was barrel-shaped and made up of thick, heavy rib bones which would have helped Hyphalosaurus stay submerged.

Hyphalosaurus appears to have exclusively inhabited deep-water lakes. All specimens are preserved in silt characteristic of the deepest part of the lake environment, and are often preserved alongside deep-water fish and crustaceans. Hyphalosaurus is also conspicuously absent from the aquatic sediments of the Jiufotang Formation, which preserved a more swampy, shallow-water ecosystem.